1. Field of the Invention
The present invention relates in general to exhaust systems of a multi-cylinder internal combustion engine, that have a catalytic converter for purifying the exhaust gas from the engine, and more particularly to the exhaust systems of a type that has main and auxiliary catalytic converters wherein when, like in a condition just after cold starting of the engine, the main catalytic converter is not sufficiently activated, the exhaust gas is led to the auxiliary catalytic converter that is easily activated.
2. Description of the Related Art
Usually, in motor vehicles powered by an internal combustion engine, the exhaust system of the engine is arranged beneath a floor of the vehicle and has a catalytic converter installed therein. If the catalytic converter is arranged in a relatively downstream position of the system, the converter takes a long time to be heated to a sufficient level for the gas purifying operation thereof particularly in a condition just after cold starting of the engine. That is, for such long time, the catalytic converter fails to exhibit its normal gas purifying work. However, if, for solving the above-mentioned drawback, the catalytic converter is arranged in a relatively upstream position of the system, that is, a position near the engine, another drawback tends to arise wherein due to attack of heat of the engine and the highly heated exhaust gas from the engine, the durability of the catalytic converter is lowered.
In order to solve the above-mentioned drawbacks, various measures have been proposed, one of which is disclosed in Japanese Laid-open Patent Application (Tokkaihei) 5-321644. In this measure, a main passage extends from an exhaust manifold of the engine to a main catalytic converter. A bypass passage having an auxiliary catalytic converter installed therein extends from an upstream part of the main passage to a downstream part of the same. A switch valve is arranged at the upstream part of the main passage to open and close the main and bypass passages selectively, and a controller is connected to the switch valve. In operation, just after cold starting of the engine, the controller controls the switch valve to introduce the exhaust gas from the engine into the bypass passage.
Since, in this measure, the auxiliary catalytic converter is positioned at a relatively upstream portion of the exhaust system, earlier activation of the auxiliary catalytic converter is expected, which induces earlier exhaust gas purifying work by the exhaust system.